You remind me of how we used to discuss film limits along the way to the sweet spot. General film wisdom landed in saying that a lens resolves optimally within 2 stops of the middle of the aperture range. This actually arrives at a 4 stop range, with the "universal optimum" for 35mm being F/8. That still basically applies, as this relates to lens performance more that the medium in use. Your criteria will vary according to circumstance, as you'll prefer a smaller aperture for macro, and a larger one for portraiture. The old rule of thumb still holds true. If the aperture range of your lens is F/2.8~F/32, the sweet spot of the lens is very near the middle. Stay within F/4~F/16 as the working range for that lens, and you'll be sure to be "safe". Use the extremes only when you must, and expect to pay a little for it.